This invention relates generally to electromagnetic flowmeters, and more particularly to a wiring harness associated with the electrodes of the flowmeter to balance out spurious signals induced in the electrode leads by the magnetic field whereby the signals carried by the leads are only those induced in the electrodes by the flow of the fluid being metered.
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,783, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul. The flowmeter can be used to meter liquids without regard to heterogeneous consistency.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
Typical of commercially-available electromagnetic flowmeters is that unit manufactured by Fischer & Porter Co. of Warminster, Pa. whose Model 10D1430 flowmeter is described in Instruction Bulletin 10D1430A-1-Revision 4. This meter consists of a carbon-steel pipe stool flanged at both ends and serving as a meter body. Saddle-shaped magnetic coils are fitted on opposite sides of the inner surface of the meter body, the magnetically-permeable pipe spool acting as a core or return path for the magnetic field generated by these coils.
The coils in this known form of meter are potted within an epoxy-based compound. An interior liner of neoprene or similar insulating material is inserted within the pipe and turned out against the faces of the mounting flanges. Disposed at the diametrically-opposed positions within the central portion of the meter body are two cylindrical electrodes that are insulated from the pipe the faces of the electrodes being flush with the inner surface of the pipe and coming in contact with the fluid to be metered. Connected to these electrodes and housed in a box external to the pipe are calibration components and a pre-amplifier.
In order to provide a compact and readily installable electromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, the Schmoock U.S. Pat. No. 4,253,340 disclosed a highly compact flowmeter which, in spite of its reduced volume and weight, is capable of withstanding high fluid pressures.
In the Schmoock flowmeter, use is made of a nonmagnetic metal spool of high strength whose inner surface is lined with insulating material to define a flow conduit for the fluid to be metered. The spool also serves to withstand fluid pressure as well as the compressive forces to which the meter is subjected by bolts bridging the flanged ends of upstream and downstream pipes between which the unit is interposed.
Surrounding the Schmoock spool and concentric therewith is a cylindrical housing formed of ferromagnetic material. The housing is provided with annular end plates that are joined to the corresponding end flanges of the spool to define an inner chamber. Integral with the housing are two magnet cores which are placed at diametrically-opposed positions along an axis which is normal to the longitudinal axis of the housing coils being wound on these cores. A pair of electrodes are mounted on the spool at diametrically-opposed portions along a transverse axis at right angles to the core axis. The inner chamber is filled with a potting compound to encapsulate the electromagnets and the electrodes, the housing serving as a mold for this purpose.
In operation, the electromagnets are excited by a pulsatory wave which establishes a magnetic field in the flow conduit that is intercepted by the fluid flowing therethrough to induce a signal in the electrodes which depends on the flow rate, this signal being conveyed by electrode leads to an external converter on secondary.
In order to prevent spurious signals from being induced in the electrode leads, it is essential in an electromagnetic flowmeter that the leads, when they run individually from the electrodes, be located as precisely as possible in a plane containing both electrodes and the axis of symmetry of the electromagnetic assembly. When the individual leads are so oriented, then the voltages induced in the two leads will be of equal amplitude and of opposite phase, so that the effect of the magnetic field in the electrode leads is balanced out; hence in the absence of fluid flow, there will be a zero signal output. It is also essential that the leads be electrostatically shielded to prevent stray pick-ups.
Within the tight confines of a Schmoock type electromatnetic flowmeter, there is insufficient room to run individual leads in the required plane to cancel out the influence thereon of the magnetic field. While it is possible to compensate for the spurious signal produced by the magnetic field, this complicates the arrangement and adds to the cost of the flowmeter.